Narcolepsy: A neurological disorder that is marked by the recurrent, sudden, uncontrollable compulsion to sleep. Narcolepsy is often associated with cataplexy (a sudden loss of muscle tone and paralysis of voluntary muscles associated with a strong emotion), sleep paralysis (immobility of the body that occurs in the transition from sleep to wakefulness), hypnagogic hallucinations (presleep dreams), and automatic behaviors (such as doing something 'automatically' and not remembering afterward how one did it). The causes of narcolepsy are unknown. Narcolepsy is not a fatal disorder in itself, but it can lead to fatalities. For example, affected persons may fall asleep while driving. Also known as excessive daytime sleepiness, hypnolepsy, sleeping disease, paroxysmal sleep, and Gelineau syndrome.